Packaging for various foods using molded articles such as polyolefin resin films or containers is now used for general purposes as an indispensable packaging means.
In addition to the basic requirements for packaging using molded articles of polyolefin resin, such as improvement of appearance of foods, prevention of adhering of bacteria to foods, and prevention of drying of foods, there has been a demand for packaging to serve to maintain freshness of foods, and much study is being given packaging materials meeting such a demand. For example, from the standpoint that freshness of foods can be maintained by using sterilized packaging films or containers, various studies have been directed to incorporation of antibacterial agents or antifungal agents into raw resins of packaging films or containers.
On the other hand, various antibacterial agents which can be added in raw resins have been proposed in the field of general food packaging. When it is considered that an antibacterial agent dissolved out from a packaging film or container would contaminate the food, nonmigratory substances, e.g., antibacterial zeolites which contain a metal having antibacterial activity such as silver, are regarded favorable.
However, in the case of the film or container obtained by molding a resin composition having compounded therewith the antibacterial zeolite powder, most of zeolite powders are embedded in the resin, so that they do not come to exhibit antibacterial activity, and zeolite powders present only in the neighborhood of the surface of molded article exhibit antibacterial activity. Therefore, in order to get the desired antibacterial activity, a large amount of zeolite powders was required to be added therein. However, there are problems that the addition of a large amount of rigid inorganic powders into the resin material lowers not only appearance of molded articles such as transparency and surface gloss, and physical property such as strength, but also injures the surface of molded article due to friction because the surface is roughened by the inorganic powders. Further, the stretch-packaging film thus obtained has serious disadvantages such as greatly reduced self-adhesion required as a stretch-packaging film.
The present inventors have found that as an antibacterial agent being capable of solving such problems of zeolite powders a silver salt of carboxylic acid and a silver salt of an alkyl ester, a phenyl ester, or an alkylphenyl ester of phosphoric acid or phosphorous acid are effective, as disclosed in Japanese patent application Nos. 1-211069 and 1-211070. However, the addition of a large amount of such organic antibacterial agents into resin materials is liable to change physical properties in the surface of molded articles due to bleedout.
Since an antibacterial agent tends to influence inherent physical properties of resin materials and it is generally expensive, if it can give sufficient antibacterial activity, it is preferred that the content thereof be small.
A corona discharge treatment intends to increase the affinity of printing inks, paints, or adhesives, etc., to the surface of non-polar polyolefin resin molded articles, namely, it is widely used to improve the adhesion properties of resins to be laminated, the deposition properties of metals to be deposited, or the adhesion properties of paints, printing inks.